Tunnel-type cryogenic freezers have been used heretofore primarily for freezing fruits, vegetables, and pizza. Food products conveyed through tunnel-type cryogenic freezers are directly cooled and/or frozen by contact with carbon dioxide snow and carbon dioxide gas. In some fruit, vegetable, and pizza freezing systems, tunnel-type CO.sub.2 freezers have been installed upstream of existing mechanical freezers (e.g., existing spiral-type freezers using coolants such as ammonia) in order to provide increased system capacity.
A typical tunnel-type cryogenic freezer will consist of: an elongate freezer housing having a warm food product inlet at one end thereof and a cooled food product outlet at the other end thereof; a series of fans and CO.sub.2 spray headers contained within the freezer housing; and a conveyor assembly for conveying the food product through the freezer housing from the inlet end to the outlet end thereof. The conveyor assembly will typically include an open conveyor belt (e.g., a flat wire, flexible, open belt) which will allow carbon dioxide snow and gas coolant to flow therethrough.
The CO.sub.2 delivery valves commonly used in tunnel-type freezers are "Praso" valves of the type disclosed in U.S. Pat. No. 3,815,377, the entire disclosure of which is incorporated herein by reference. Praso valves are commercially available, for example, from Robert Shaw Company, Knoxville, Tenn. and from other commercial manufacturers. During operation, each Praso valve will have a Praso valve flow nozzle provided therein. The particular Praso valve nozzle employed will determine the velocity and flow pattern of the CO.sub.2 delivered by the valve. The nozzle will also determine the degree to which the delivered CO.sub.2 impinges upon the food product being conveyed through the tunnel freezer.
It has been common practice heretofore to use identical fan blades, identical constant speed fan motors, identical Praso valve nozzles, identical Praso valve set pressures (typically about 100 scfh at 235-245 psig), and identical Praso valve/conveyor belt clearances throughout the entire length of the tunnel freezer. Thus, the degree to which the delivered CO.sub.2 impinges upon the conveyed food product, the delivered ratio of CO.sub.2 snow to CO.sub.2 gas, and the degree of fan turbulence encountered by the conveyed food product has been substantially constant along the entire length of the tunnel freezer.
It is also noted that CO.sub.2 supply pressure has heretofore been maintained at about 260 psig by controlled evaporation of CO.sub.2 in the CO.sub.2 supply system.
Some attempts have been made heretofore to use tunnel-type CO.sub.2 freezers for fully freezing chicken products. In an effort to obtain adequate heat removal while minimizing CO.sub.2 consumption per pound of product, high impingement (i.e., 30 mm barrel length) Praso valve nozzles and high fan circulation rates have been employed along the entire length of the freezer.
Unfortunately, the results obtained in these prior efforts have been largely disappointing, particularly when the chicken products in question were products having marination fluid and/or skin on the exterior thereof. Contrary to expectations, product dehydration and CO.sub.2 consumption levels have been excessive. Moreover, the high impingement and turbulence levels employed in these prior efforts have caused skin loss, skin breakage, skin finger formation, marination fluid loss, and skin bubbling. Skin bubbling problems have been especially troublesome in the case of injection marinated products. In such applications, the needle injection hole formed during the marination injection process provides an opening through which impinging CO.sub.2 can readily flow beneath the product skin.
Prior attempts at using tunnel-type CO.sub.2 freezers for freezing chicken products have also been characterized by product streaking problems. Heretofore, in chicken freezing applications, each Praso valve header extending laterally across the width of the freezer conveyor belt has had five Praso valves attached thereto. Moreover, the Praso valve headers have typically been structured and arranged in the tunnel freezer such that the individual valves connected to the various valve headers have been aligned in linear relationships along the length of the freezer. Given a standard valve to belt clearance in the range of from 4 to 8 inches and using high impingement, 30 mm barrel Praso valve nozzles of the type heretofore employed for freezing chicken products, the overall spray pattern provided by each valve header has typically directly covered only about one-half of the total width of the conveyor belt. As a result, visible streaks have been formed on the surface of the frozen product.
When using a tunnel freezer for freezing pizza, excessive impingement of CO.sub.2 coolant onto the surface of the pizza product could damage or remove the pizza toppings. Thus, in CO.sub.2 tunnel freezers used heretofore for freezing pizza, flat plate orifices (i.e., valve nozzles having barrel lengths of 0 mm) have been employed throughout the freezer in all of the CO.sub.2 valves. When flat plate orifices are employed, the CO.sub.2 coolant does not substantially impinge on the pizza product. Rather, the flat plate orifices provide a relatively gentle CO.sub.2 snow blanket which deposits on the pizza product. As a result of their use in pizza freezers, flat plate orifices are commonly referred to as pizza orifices.
Unfortunately, the use of tunnel-type pizza freezers for freezing chicken products would be economically prohibitive due to the fact that several pizza freezers, aligned in series, would be required to obtain an adequate amount of cooling.